1. Field of the Invention
The present invention relates to a road paving material. More particularly, the present invention relates to an asphalt based road paving material having improved water permeability and sound absorption.
2. Description of the Related Art
Past paving materials used on sidewalks, vehicle roads, and park walkways, for example, have been made of tile, concrete, asphalt, and the like. These materials are not water permeable. As a result, most of the rain water falling on them is not absorbed into the ground, but runs directly into the sewer system. This creates problems such as a shortage of underground water, lower ground levels, reduced tree and shrub growth, and the like.
To remedy these problems, attempts have been made to increase the water permeability of the paving materials by mixing with the asphalt an aggregate of comparatively large stones and the like having grain diameters in the range from about 2 millimeters to about 12 millimeters. The aggregate creates large numbers of continuous pores within the asphalt to pass water. See, e.g., Official Publication of Toku Kai Sho 57-140401.
With these water-permeable paving materials, however, it has been found that with the passage of time, the aggregate tends to become scattered or the pores become clogged by dust, thereby decreasing water permeability.
It has also been found that with time, the plasticizing substance, e.g., the oil portion of the asphalt, evaporates and hardens, which lowers the asphalt's so-called “cushion effect” and lowers the aggregate's binding force. This, in turn, makes the asphalt less stable for vehicles to travel on and less comfortable to walk on. The asphalt is thus susceptible to changes in temperature, e.g., the alternating heat and cold of the seasons causes cracking. Moreover, during the winter this causes pot holes, and leads to a significant increase in wear due to vehicles equipped with snow tires, spike tires, and tire chains. And during the summer, the paved roads tend to become fluidized or softened by high temperatures, thereby creating cracks, stickiness, and the like.